Re: [PATCH 13/16] mm/migration: return errno when isolate_huge_page failed

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/3/7 10:14, Baolin Wang wrote:
> Hi Miaohe,
> 
> On 3/4/2022 5:34 PM, Miaohe Lin wrote:
>> We should return errno (-EBUSY here) when failed to isolate the huge page
>> rather than always return 1 which could confuse the user.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>   mm/migrate.c | 6 ++----
>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 6c2dfed2ddb8..279940c0c064 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1618,10 +1618,8 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>>           goto out_putpage;
>>         if (PageHuge(page)) {
>> -        if (PageHead(page)) {
>> -            isolate_huge_page(page, pagelist);
>> -            err = 1;
>> -        }
>> +        if (PageHead(page))
>> +            err = isolate_huge_page(page, pagelist) ? 1 : -EBUSY;
> 
> Could you elaborate on which case the huge page isolation can be failed in this case? Or you met a real problem? Cause now we've got this head huge page refcnt, I can not see why we'll fail to isolate this huge page.

IIUC, this could happen when hugepage is under migration which cleared HPageMigratable. Page refcnt cannot
prevent isolate_huge_page from happening. Or am I miss something?

Many thanks.

> .